A polyphase AC motor is used in various applications, including an electrically assisted power steering apparatus. The power supplied to the polyphase AC motor is controlled by an inverter apparatus for polyphase AC motor drive. The following explanation takes a three-phase AC motor as an example of the polyphase AC motor. An electrically assisted power steering apparatus reduces the required steering torque applied by a driver, which is configured by combining a power source, a motor, a sensor, an inverter apparatus for three-phase AC motor drive and a controller, and which is a system that uses the sensor to detect the direction and amount of the rotation angular velocity of a steering torque applied by the driver, then uses the inverter apparatus for three-phase AC motor drive to transform power from the power source to a required amount of power to be supplied to the motor, and then uses the motor to add an assist torque to the steering torque.
For the electrically assisted power steering apparatus in which the inverter apparatus for three-phase AC motor drive is used, conventionally, various measures against a failure occurring in the motor or inverter apparatus have been considered. Also, various failure detection methods have been proposed. When a failure is detected, a portion in which the failure has been detected will be isolated from the overall circuit configuration. In order to do that, for example, one method has been proposed in which a motor relay is provided between the U-, V- and W-phase coils of the motor and the U-, V- and W-phase arm circuits of the inverter circuit, respectively, for separating the inverter circuit from the faulty motor, and a power supply relay is provided between the invert apparatus and the power source for cutting off power supply. Indeed, quickly stopping the motor would be a measure against an overall systematic failure, but, stopping the motor causes a problem of largely changing the steering characteristics and requiring the driver to steer with more steering torque.
Therefore, a PTL 1 proposes a method in which, when a failure occurs in only one phase of the three phases of the motor, the phase in which the failure has occurred is determined in order to use the phases other than the faulty phase as an energized phase to continue motor drive and continue to add an assist torque to the steering.
As a premise of the proposal, it is disclosed that, when only one phase of the three-phase (U, V, W) motor coils is insufficiently energized, the two phases other than the insufficiently energized phase are used as an energized phase to continue motor control.